As even novice audio recordists know, a stereo file consists of two channels: left and right. Signal that is routed simultaneously and at equal level to both channels appears as a ‘phantom’ center image, while signal that is panned unequally to the L/R channels appears to the left or right of center.

What you may not know is that modern digital algorithms are capable of further separating stereo content into a monophonic mid channel (sometimes called a “center” channel) and a stereo sides channel. This means that you can apply equalization, compression or other effects to the sounds in the middle independently of the sounds panned to the sides – a technique called mid-side (MS) processing.

Bet now you’re interested. Here are five basic concepts about MS processing that will get you up to speed:

(Important: What we’re discussing here is not the mic technique of a similar name, where an array of two microphones at right angles – one of which is inverted in phase – is used to record a sound source in stereo while preserving mono compatibility.)

1. It’s not just a mastering tool… Try it in your mix

MS processing has become routine for many engineers during the mastering stage, but it can really come in handy during mixing too. In mastering, it’s usually used to reach into a flawed mix and make necessary changes in balance and width, but you can achieve the same results by applying a plugin that converts a L/R signal to mid-side (such as Waves Center) to the master buss on your mix. Just be sure not to do any MS processing until after you have all the elements balanced across the L/R stereo field. Trying to do so during mixing will make your head spin!

Of course, you can also apply MS processing to individual stereo sources such as sub-busses, stems, synths or loops. Doing so gives you the ability to alter or enhance without affecting the entire mix.

Whether you use it in mastering or mixing, MS processing can change the width and depth of a track in a variety of subtle, effective ways:

Rework the balance between lead vocal and stereo background vocals.

Manipulate mono and stereo percussion instruments within a drum loop.

Tame or push hard-panned elements.

Change the apparent front-to-back depth by making ambient elements (such as drum overheads or reverb) come forward or recede into the mix.

2. Start with simple level adjustments

Before you begin getting fancy with MS processing – and there are plenty of ways to do so – start with the simplest usage of all: Play with the relative levels of the mid and sides channels. This simple operation can bring focus and stability to the center, and excitement to the sides.

Boosting the sides channel by just 1 or 2 dB can work wonders in certain situations, giving the track a wider, more open sound while at the same time gently making cymbals, guitars and other elements panned to the left or right become a little more alive. The vocals tend to sit in the track a little better, too.

Conversely, dipping the sides a bit lower (or raising the mid channel a hair) will strengthen the track overall by making everything more mono. This is especially true for the low frequencies, which typically live predominately in the center, as well as for making the center or lead vocal more prominent.

So if you have a track that sounds narrow overall, insert Waves Center, turn down the center channel a little, then turn up the sides a little. Instant extra width! If you have a track that is wide and needs some overall tightening and focus of the stereo image, try dipping the sides and up the center channel to taste.

Note that when the mid and sides channels are at equal levels, you’ll hear the mix in its original form – exactly as it was before being converted to an MS signal.

In this video example, you can hear how boosting the sides channel and attenuating the center instantly adds width to an already-stereo track:

3. Use equalization on the mid and sides channels

Being able to independently equalize the mid and sides portions of your mix can yield some incredibly cool and useful results. For example:

Try bussing instruments separately from vocals and solos and insert a mid-side EQ plugin such as the Waves Scheps 73 or Abbey Road RS56 Passive EQ. Do a frequency sweep to locate the frequencies of the vocals or solo that give them their power, then cut that frequency on the mid channel of the instrument buss. Voila! The lead vocal or solo will poke through with a degree of clarity that you could not achieve with a normal EQ, since you would otherwise be cutting those frequencies from the entire stereo spread. Cutting across the full image would be a poor solution, since the side information is very important for maintaining fullness and width.

On your mix buss, boost the upper-mids of the sides to add shimmer to stereo strings or cymbals without making the lead vocal or guitar solo too harsh – all while leaving the core elements (kick, snare, bass) untouched. Conversely, try applying a little low-frequency boost to the mid channel to bring out just the kick drum and bass guitar (both of which typically live only in the center) without muddying the stereo signals in the mix.

Say you’re working on a track where the stereo piano is clashing with the lead vocal in the 1kHz - 2kHz area. Try sending the piano’s tracks to a separate buss and remove some 1kHz content on its mid channel to make space for the vocal, and boost 3k - 4k in the sides channel to compensate.

If you want to add some presence to a stereo sound and help it cut through the center of the mix, you can boost upper-mid frequencies on its mid channel, and, at the same time, add some warmth by filtering out the highs on its sides channel.

Try selectively de-essing either mono or stereo vocals that are buried in a mix.

Apply a peaking filter to the sides channel to reduce harshness in cymbals. Or try scooping some of the mid-range frequencies in the mid channel (say around 250 Hz and/or around 2 kHz) so they can come through more easily in the sides channel. This kind of subtractive equalization in the mid channel will not only leave more room in your mix for your main lead and bass elements, but it will also create more separation from the side channels, adding space and air as well as increasing the perceived width.

The following example shows how to give a stereo track extra width and have it stand out from within a dense mix:

4. Use MS for selective compression

The PuigChild Compressor, modeled on the famed Fairchild 670 compressor, offers separate compression for the mid and sides channels, referred to on the original unit as vertical and lateral.

Get things pumping on your effects return by applying a compressor to the vertical, or mid channel only with a fast attack time to squash the transients – a technique that greatly reduces the risk of artifacts on the reverb and room ambience, both typically panned to the outer edges of a mix. If you compress both mid and sides channels on a reverb buss, compressing the mid channel more than the sides will have the effect of making the reverb wider.

Or, on your mix buss, try compressing the sides channel to bring out the softer, textural elements. This will serve to accentuate the ambiance without overly squashing the backbeat – sort of like compressing the room mics on a drumkit. When compressing the lateral, or sides channel, you’ll want to just ‘kiss’ the gain reduction by a dB or two and use fairly slow attack/release times so that the transients (i.e., cymbals, high-hat) get through; otherwise, things will start sounding very unnatural. (Although, on second thought, that might be exactly the effect you’re after!)

5. MS processing + Dynamic EQ = Killer combination

Dynamic equalizers such as the Waves F6 Floating-Band Dynamic EQ are powerful tools that provide precision equalization along with band-specific compression/expansion and sidechain triggers, allowing you to craft EQ curves that move with the music. What’s more, each band of the F6 can process a signal in left/right stereo or in MS mode, enabling you to EQ the center of a mix – the singer, for example – without affecting the color or shape of the overall stereo image.

Suppose some side-panned rhythm guitars are not prominent enough in the mix. An obvious solution would be to try to bring them out a little by adding some midrange EQ. However, if you apply this across the left-right mix, you’ll likely end up thickening in the lead vocal’s range – probably not a good thing. Much better to work with the MS signal instead and add some subtle EQ and compression to selective frequencies in the sides channel only, along with possibly a slight complementary cut to the mid channel.

Or let’s say that some instruments and the lead vocal are competing for the same space in a mix – a pretty common problem. With a dynamic equalizer like the F6, you can carve out a spot for the lead vocal without affecting the stereo width of your mix while at the same time maximizing the blend, or ‘glue,’ between the instruments and the lead vocal.

Alternatively, you can use the snare or kick as the sidechain source to create momentary frequency “duckings” that allow other instruments (such as bass guitar) to poke through. If you’re working in the EDM genre, you can tame aggressive white-noisey synth sounds in sides channel by ducking their high-frequency content whenever handclaps, cymbal crashes, or other transient sounds panned to the center are heard – a great way to transparently open up the top end.